Emergency service calls in North America are originated by dialing “9-1-1.” Other parts of the world may use some other abbreviated string of dialable digits such as “6-1-1” in Mexico; all share the intent to provide the caller with an easy way to call for help with an easy to remember number. These calls are routed to a local Public Service Answering Point (PSAP) where an emergency response may be initiated (police, fire department, road repair, ambulance, etc.) while the caller is kept on the phone. If the call is somehow disconnected or dropped before the emergency is completely reported or the responder arrives, the PSAP may call back the originator using a call back number provided through its databases.
For example, the call record for a 911 call originated through a wired network may include Automatic Line Identification (ANI) or the telephone number of the access line from which the call originated. However, the mobile directory number (MDN) or telephone number of a wireless subscriber is not associated with a physical line or mobile station. Instead, calls to a wireless subscriber are routed to the mobile station by way of the mobile station identification (MSID), not the MDN. Accordingly, performing an emergency call back to a mobile station poses hurdles not encountered with, e.g., land line devices.
Typically, the MSID is either a 10-digit mobile identification number (MIN) or a 15-digit International Mobile Subscriber Identifier (IMSI) programmed into a mobile station by the service provider with whom the mobile station user has entered into a service agreement. Accordingly, the MSID is not necessarily a dialable number.
The MDN of a mobile station is a dialable number. The MDN is dialed by a caller and used to route a call through the network to the wireless subscriber's home system. At the subscriber's home system, the home location register (HLR) contains the MSID associated with the subscriber's MDN. The MSID, not the MDN, is then used to route the call through the network to the serving wireless system and page the subscriber. The subscriber's MDN is provided by the home system to the serving system in a separate data file called the subscriber profile.
The use of a separate number for MDN and MSID is new to some systems. Historically, in TIA/EIA-41 systems before the implementation of wireless number portability (WNP) or thousands block number pooling (TBNP) based on the Local Routing Number (LRN) method and international roaming (IR), the mobile identification number (MIN) of a mobile station was the same as the MDN. However, with WNP and TBNP, the MDN became “portable” or “poolable” from one service provider to another service provider. Since MSID is not portable or poolable, the recipient service provider assigns a new MSID for a subscriber with a ported-in or pooled MDN.
International roaming also forced the separation of MSID and MDN. While the MIN is a 10-digit number modeled after the North American Numbering Plan's 10-digit MDN, other nation's carriers using a different directory numbering plan may not allow their MDN to be equivalent to the internationally recognized MIN format. Another standard MSID is the IMSI. It is used in both TIA/EIA-41 and GSM systems around the world. IMSI is a 15-digit number, and therefore, can not serve as a 10-digit MDN.
Historically, when the MDN was the same as the MIN, the MIN would be delivered to a PSAP and would be used for a call back number. With the separation of MIN and MDN as described above, it became necessary to deliver the MDN as a separate call back number to the PSAP as well as the caller's MSID. There are certain problems associated with implementing this solution. The primary problem is that the serving system may not have the caller's MDN, only the MSID, to present to the PSAP with the call. Some of the reasons for this relate to the way MSID-MDN separation has been implemented according to standards.
An old serving TIA/EIA-41 system may not support WNP, TBNP or IR. This means that the older serving system may be expecting the MIN and the MDN to be the same. The older system would not even know to look for a separate MDN in the subscriber's service profile (keyed on MIN, not MDN). With this limitation, these subscribers may not be allowed to use basic services, but they must be allowed to call for emergency services. As a result, a roamer who dials “9-1-1” while on an old system will have his or her call delivered to the PSAP with an MSID but no MDN. Accordingly, no call back is possible.
A newer serving system that is WNP and IR capable may not be able to deliver MDN to the PSAP. This could happen if the calling mobile station is not registered with any service provider (e.g., there are mobile phones used for emergency calls only). It is also possible for a subscriber to place an emergency call before the HLR has responded to the serving system with the subscriber's service profile containing the MDN.
The call back MDN for an international roamer would require the PSAP to place an international call to reach a subscriber in their local Emergency Service Zone (ESZ). This is not a practical, timely or sufficiently reliable solution for a PSAP that normally does not place international calls and may require immediate call back information in order to save someone's life. In addition, the entire international MDN (up to 15 digits including a country code) may not be presented to the PSAP for callback.
One proposed solution to these problems calls for delivering 9-1-1+the last 7 digits of the electronic serial number (ESN) of the calling mobile station to the PSAP as the call back number when the MDN is not available. While this may serve to identify the caller to the PSAP and the serving system, this “9-1-1+ESN7” can not be routed through the network and can not be used to place a call back.